performance issues in simple arithmetic code
rtvd at mac.com
Thu Apr 28 10:41:48 CEST 2011
>> Question: can't it use cheap multiplication and shift instead of expensive
>> division here? I know that such optimisation is implemented at least to some
>> extent for C--. I suppose it also won't do anything smart for expressions
>> like a*4 or a/4 for the same reason.
> There isn't really any optimisation done on Cmm and the native code
> generator doesn't do much optimisation itself, hence you get the more
> direct forward translation. This kind of code is where the LLVM
> backend does well in comparison. I haven't tried benchmarking the
> performance of -fasm vs -fllvm for this code but if you eyeball the
> assembly code produced by -fllvm then you'll see it uses shifts and
> other magic.
Well.. I found some places in C-- compiler which are supposed to convert
division and multiplication by 2^n into shifts. And I believe these work
However in this case I am a bit puzzled because even if I change the
constants in my example to 2^n like 1024 the code is not optimised.
By the way, is there any kind of documentation on how to hack C-- compiler?
In particular, I am interested in:
* how to run its optimiser against some C-- code and see what does it do
* knowing more about its internals
With kind regards,
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